Holographic interferometry employing image plane holograms

ABSTRACT

A hologram of a test specimen is formed in an optical arrangement including a lens which focuses coherent light reflected by the object onto a photographic plate. A real-time interferometric analysis of the deformation of the object as a result of loading is made using the resulting image-plane hologram. The interference fringe frequency in particular area resulting from gross deformation of the object is decreased to allow the detection of fringe anomalies in that area by suitable translations of the hologram relative to the reconstruction beam. In an alternate embodiment, a pair of image-plane holograms of the object at two states of loading are formed on the same photographic plate by a double-exposure technique using reference beams which bear different angles to the photographic plates during the two exposures. To reconstruct the holograms a pair of reconstruction beams are employed and motion of the beams relative to one another allows the fringe frequency on various areas of the object to be modified.

xrz 3,690,159

ll'nite SE31 Kersch et a1.

[54] HOLOGRAPHIC INTERFEROMETRY 'EMPLOYING IMAGE PLANE HOLOGRAMS [72]Inventors: Leonard A. Kersch; Edwin B. Champagne, both of Ann Arbor,Mich.

[731 Assignee: GC Optronics, llnc.,

Mich.

221 Filed: April 29,1970

21 Appl. No.: 32,941

Ann Arbor,

[52] US. Cl ..73/88 A, 73/15.6, 73/7l.3, 350/35, 356/32, 356/109 [51]Int. Cl. ..G01b 11/16, GOln 3/18 [58] Field of Search ..73/67.5 H, 71.3,15.6, 88 A; 350/3.5; 356/106, 109, 113, 32

[56] References Cited OTHER PUBLICATIONS Hologram Interferometry,Stetson et al. J. O. S. A. Vol. 56, No. 9, September 1966, pg. 1161-1166. Holographic Multiple- Beam Interferometry, Matsumoto J.O.S.A. Vol.59, No. 6, June 1969, pg. 777- Focused-Image Holography with ExtendedSources, Rosen Applied Physics Letters, Vol. 9, No. 9, Nov. 1, 1966 pg.337- 339 Hologram Interferometry Using Two Reference Beams, Tsuruta etal., Japanese Journal of Applied Physics, Sept. 1968, pg. 1092- 1100 1Sept. 12, 1972 Double-Exposure Holographic Interferometry with SeparateReference Beams, Ballard, Journal of Applied Physics, Vol. 39, No. 10;September 1968, pg. 4846-4848 Primary Examiner-Richard C. QueisserAssistant ExaminerJohn P. Beauchamp Attorney-McGlynn, Reising, Milton &Ethington [57] ABSTRACT A hologram of a test specimen is formed in anoptical arrangement including a lens which focuses coherent lightreflected by the object onto a photographic plate. A real-timeinterferometric analysis of the deformation of the object as a result ofloading is made using the resulting image-plane hologram. Theinterference fringe frequency in particular area resulting from grossdeformation of the object is decreased to allow the detection of fringeanomalies in that area by suitable translations of the hologram relativeto the reconstruction beam.

In an alternate embodiment, a pair of image-plane holograms of theobject at two states of loading are formed on the same photographicplate by a doubleexposure technique using reference beams which beardifferent angles to the photographic plates during the two exposures. Toreconstruct the holograms a pair of reconstruction beams are employedand motion of the beams relative to one another allows the fringefrequency on various areas of the object to be modified.

11 Claims, 7 Drawing Figures HOLOGRAPHIC INTERFEROMETRY EMPLOYING IMAGEPLANE HOLOGRAMS BACKGROUND OF THE INVENTION quantitative deformationmeasurements and more particularly to techniques which allow the fringepattern obtained in the interferometric process to be modified tocompensate for gross deformations of the object and to subjectparticular areas of the object to detailed examination.

2. Prior Art Holography involves the recording and reconstruction oflight wavefronts by photographically recording the interference patternbetween coherent light reflected from a scene or object and a referencewavefront of coherent light from the same source that illuminated thescene or object. After development and fixing, the resulting hologrammay be used to reconstruct the original reflected light wavefront byilluminating the hologram with an appropriate light source, usuallycoherent. The development of the laser as a source of highly coherentlight has spurred the development of holography in the last few years.Holographic Interferometry is a technique which employs a wavefrontsystem reconstructed from a hologram to detect minute deformations onthe surface of an object. In real-time analysis the reconstructedwavefront, representing an object at a first time, is superimposed on awavefront reflected from the object at a second time while the object isilluminated with light that is coherent with that used in thereconstruction. In double-exposure. holographic interferometry,wavefronts of the object at two separate times are both recorded on thesame photographic plate, so that they may be simultaneouslyreconstructed in the superimposed manner.

In both techniques minute deformations of the object resulting frommovements between the times of the formation of the two wavefronts arerevealed in the form of fringe lines which are visible on thesuperimposed reconstruction. These fringe lines generally are arrayedsomewhat as contours of equal displacement of the surface of the objectbetween the two times under consideration. If the entire objecttranslates normally to the line of vision of the observer viewing therecon struction through the hologram, almost no fringe line array willbe noted, (absolutely no fringe array will be noted if the wavefrontswere planar rather than curvalinear) although the total surface of theimage may appear brightened or darkened. If the object has been rotatedabout an axis parallel to the hologram between the two times underconsideration, a relatively uniform array of fringe lines will appear onthe interferometric reconstruction.

Holographic interferometry has been successfully employed tonondestructively test a variety of forms of workpieces by stressing themin some way between the times of the two wavefronts and then detectinganomalies in the deformation pattern which is revealed by theinterferometric analysis. For example, separations between the carcassand plies of a tire may be detected by inflating the tire and performingan interferometric analysis of the surface of the tire two times shortlyafter inflation. Following inflation the tire will deform for some timeas a result of creep of the rubber and sections of the surface overlyinga separation will tend to creep at a different rate than the balance ofthe tire. Accordingly, a holographic interferometric analysis made attwo times after inflation will reveal such separations.

The fringe lines which occur in holographic interferometric analysisrepresent increments in the surface deformation of the object betweenthe times of formation of the two wavefronts, and their spacing is afunc- 5 tion of the wavelength of the coherent light employed in theanalysis. If a section of the surface moves through a distance which isgross compared to this wavelength, a high fringe density results makingit difficult to detect anomalies in the motion pattern. The presentinvention has as its prime object to provide a method of realtime anddouble-exposure holographic interferometric analysis which compensatesfor gross deformation of the object of the type which might cause highfringe densities so as to allow the detection and analysis of anomaliesin these grossly deformed areas.

At the present state of the art of holographic interferometry, one ofthe advantages of real-time techniques over the double-exposure varietyis the ability to modify the illumination direction or otherwise modifythe test setup, in real time, to achieve optimal conditions fordetecting suspected anomalous areas. An object of the present inventionis to provide a method wherein the manner in which a double-exposurehologram is reconstructed may be modified to provide similar detailedanalysis of various sections of the workpiece.

SUMMARY OF THE PRESENT INVENTION The present invention contemplatesnovel processes of both real-time and double-exposure holographicinterferometry. The holograms used in both of these techniques differfrom conventional holograms in that a lens or lens system is disposedbetween the object and the photographic plate so as to focus an image ofthe object on the plate much in the manner of conventional photography.In the more conventional holography the object is not imaged on thephotographic plate but rather every point on the photographic plate isexposed to lightwaves reflected from every point on the object which maybe connected with that point on the photographic plate by a straightline. In essence, the present image-plane holograms do not record thelight waves emanating from the object but rather record the lightwavefronts emanating from the system consisting of the object and thelens. Upon reconstruction the same image of the object is seen as wouldbe seen through the lens. Such an image-plane hologram is a hybridbetween a conventional hologram and a conventional photograph andexhibits certain properties of each. With a conventional hologram thewavefronts produced on reconstruction will be a distorted version of theoriginal wavefronts if the angle of the reconstructing beam with respectto the hologram differs from the angle that the reference beam bore tothe photographic plate during the formation of the hologram. Of course,in a conventional photograph no particular angular relation between thephotograph and the illuminating light source may be observed. Theimage-plane hologram resembles the conventional photograph in thisproperty. Since the reconstructed image occurs in the plane of thehologram plate, by varying the angle of the hologram with respect to thereconstruction light source, the position of the reconstructed imagechanges but its proportions do not become distorted.

In the simplest embodiment of the present invention an image-planehologram is formed of an object and later distortion of the visiblesurface of the object is measured by reconstructing the original imagefrom the hologram and superimposing it upon the actual image of theobject as seen in real time. With the hologram in exactly the sameposition as the photographic plate occupied during formation of thehologram, the image seen through the hologram will appear with fringelines superimposed thereon resulting from the interference between thetwo images. These fringe lines will be the result of the deformation ofthe object during the time since the formation of the original hologram.Certain deformations may produce such high fringe densities in localizedareas as to make it difficult to analyze the deformation in those areas.In order to modify localized fringe density it is only necessary to movethe reconstruction beam. This motion will adjust the phase of thereconstructed wavefront relative to the wavefront emanating from theobject in real time. If the original object has undergone an overalltranslation since the time of formation of the hologram, all fringesresulting from this translation can be eliminated from the analysisimage by a motion of the reconstructing wavefront. If the object hasbeen locally deformed so as to produce a high fringe density inparticular areas, making detailed analysis of the deformation of thesesareas difficult, the overall fringe density in these areas may bedecreased by a suitable motion of the hologram relative to thereconstructing beam, allowing anomalies in the deformation pattern to bemade visible and thus detectable. This motion may simultaneouslyincrease the fringe frequency in other areas of the object.

This technique makes use of the previously noted fact that a rotation ofan image-plane hologram with respect to the reconstructing light beamdoes not distort the reconstructed wavefront but only modifies itsproportions relative to the axis of viewing in the same manner as if aphotograph were rotated with respect to its viewing axis. With a normalhologram, the motion of the hologram would produce a distortion of thereconstructed wavefront which would make meaningful analysis extremelydifficult.

The method of the present invention may also be applied todouble-exposure holography by superimposing two holograms of an objectformed at spaced times, on the same plate using reference beams to formthe two holograms which bear different angles with respect to thephotographic plate. Upon reconstruction, using separate reconstructingbeams corresponding to the separate reference beams, the alignment ofthe two reconstructed wavefronts relative to one another may be adjustedby moving one of the reconstructing beams relative to the other. Thistechnique again allows for the modification of the overall fringedensity in selected areas of the object so as to allow for the detailedinspection of particular areas and the removal of fringe patternsresulting from overall motion. Were a single reference beam used in bothof the exposures, the later motion of the reconstruction beam relativeto the hologram would modify the two reconstructed wavefronts equally,making it impossible to modify the spatial frequency of the fringes inany selected area.

This double-exposure technique is particularly valuable in productionsituations since it allows a single double-exposure hologram to beformed of a large area relatively quickly and allows a later detailedexamination of selected points within that area without requiring thepresence of the actual object. This double-exposure technique also makesuse of the fact that the wavefronts reconstructed from an image-planehologram are not distorted by a modification of the angle between thehologram and the reconstructing beam.

Other objects, advantages and applications of the present invention willbe made apparent by the following detailed description of two preferredembodiments of the invention. The description makes reference to theaccompanying drawings in which:

FIG. I is a plan view of optical apparatus for forming a hologram of anobject and reconstructing a wavefront from that hologram so that it issuperimposed on the wavefronts emanating from the object in real time ata later time, useful in the practice of a first embodiment of theinvention;

FIG. 2 is a perspective view of the apparatus of FIG.

FIG. 3 is a plan view of an object which may be tested with theapparatus of FIG. 1 and 2, illustrating a deformation of the object;

FIG. 41 is an illustration of the interference fringes which would occuron the image of the object of FIG. 3 during real-time analysis inabsence of the practice of the method of the present invention;

FIG. 5 is an illustration of the fringes which appear on the image ofthe object of FIG. 3 using the method of the present invention todecrease the fringe frequency on the left side of the object;

FIG. 6 is an illustration of the fringes which appear on the image ofthe object using the method of the present invention to decrease thefringe frequency on the right side of the object; and

FIG. '7 is a plan view of apparatus for practicing a second embodimentof the invention involving doubleexposure holography.

The application of the present invention to real-time holographicinterferometric analysis preferably employs apparatus of the typeillustrated in FIGS. 1 and 2. The apparatus is illustrated as performingan interferometric analysis on a plane section of honeycomb 10 whichincludes a pair of thin metal skin sheets 12 and Ml joined together bycentral honeycomb core 16 projecting normally to the sheets. (This typeof material is employed in aircraft structures). A section of the skinsheet M is shown broken away in the lower left-hand corner to illustratethe core 16.

This test specimen It), as well as the other apparatus used in thepractice of the invention, is supported on a heavy iron plate 18 whichis suitably isolated from vibration in accordance with conventionalholographic practice. The test specimen 10 is retained on the base plate18 by a pair of upright members 20 and 22 which abut its side edges soas to prevent any lateral dimensional expansion.

The coherent light used to perform the holographic analysis is derivedfrom a laser 24 which projects its beam to a half-silvered mirror 26acting as a beam splitter, to form a pair of beams which are coherentwith respect to one another. One of the beams is passed through aspatial filter 23, comprising a lens and pinhole assembly operating toeliminate the spread edges of the beam and to form the beam into adiverging pattern. The beam is then reflected by a mirror 30 to aphotographic plate 32 which is retained in a plate holder 34, where itacts as the reference beam in the formation of the hologram. The otherbeam from the half-silvered mirror 26 is reflected by a mirror 36through a spatial filter 38 and is then reflected by a mirror 40 so asto project it upon that surface of the workpiece lltl which is opposedto the photographic plate 32. A certain portion of the light reflectedfrom the workpiece reaches an imaging lens 42 which has a focal distancesuch as to focus the illuminated surface of the workpiece on to theplate 32.

Utilizing a photographic film of suitable sensitivity and grain size,and exposure times which are now wellknown to those who practiceholography, the photographic plate 32 is exposed to record theinterference pattern between the reference beam reflected from themirror 30 and the object beam produced by the lens 42. Upon suitabledevelopment of the photographic plate, which may either be performed ina situ or at a remote location, a hologram is formed from which thewavefronts reaching the photographic plate 32 from the lens 42, duringthe exposure process, may be reconstructed.

The hologram thus formed is conventional in the sense that the objectwavefront may be reconstructed from it, but is unconventional in thatthe object being reconstructed constitutes a wavefront containing theimage of the ultimate object 10 as focused by the lens 42. Were the lens42 to be removed from the system, every point on the object 10 facingthe photographic light would reflect to every point on the photographicplate. The lens 42 focuses the beam on the plate 32 so that there is aone-to-one correspondence between the points on the illuminated surfaceof the object 10 and the surface of the photographic plate 32illuminated by the object beam. Such an image-plane hologram haspreviousiy been known to the art but not as part of the presentinvention.

The developed hologram may be reinserted in the plate holder 34 in thesame position previously occupied by the photographic plate. With theobject in exactly the same position as previously, the light wavefrontsforming the reconstructed image of the object will coincide with thosewavefronts actually emanating from the object in real time, and only oneobject will be observed by an observer looking through the hologram fromthe right side of the plate as viewed in FIGS. 1 and 2. If the objecthas been grossly displaced since the time that the original hologram wasformed, such as by a movement through one-half inch, dual images of theobject, not exactly superimposed, will be seen through the hologram.

if, however, sections of the object have been deformed throughrelatively small distances, such as 0.0001 inch, as might be caused by achange in ambient temperature inducing thermal stresses in the object,only a single object will be observed but it will appear to containfringe lines which are the result of the interference between thereal-time and the reconstructed light wavefronts. The lines in FIG. 1 tothe right of the plate holder 34 illustrate the manner in which thereconstructed wavefronts 41b and the real-time wavefronts 46' interfereby reason of a motion of the workpiece between the time of formation ofthe original hologram and the later reconstruction, The angles shown inFIG. ii are purposely exaggerated for better illustration.

in order to test some property of the object it), such as the bondbetween the skin 14 and the core 16, the workpiece 10 may be purposelystressed during the time between the formation of the original hologramand the later real-time viewing. This might be done by a heat lamp 48which projects its beam on the face of the workpiece 10. The resultantheating causes an expansion of the skin M which causes it to pull awayfrom the core 116. This deformation caused by the heating will result inoverall displacement of the object 10 but will also result in anomalousdisplacement of any sections of the skin 14 adjacent to areas which arenot firmly adhered to the core 60. in this manner the bond between theskin and the core may be tested.

A difficulty encountered in this test procedure is that the fringesresulting from overall deformation of the workpiece 10 act to mask thelocalized deformation anomalies resulting from poor bonds. FIG. 3illustrates the overall deformation which might occur in the object as aresult of the heating by the lamp 48. Because of the restraint imposedby the uprights 20 and 22, the expansion of the honeycomb from theheating may cause it to bow to the position FIG. 4 illustrates thefringe families that might be visible on the superimposed reconstructedand realtime wavefronts in the real-time analysis process on thereconstructed image of the workpiece 10''. The bowing of the workpiece110 has been assumed to be uniform along vertical lines for purposes ofthis illustration. The fringes have a spatial frequency which is afunction of the slop of the workpiece with respect to its original planeconfiguration. Since the ends are constrained they exhibit relativelylow spatial frequencies, and the middle is largely translated so itexhibits a low spatial frequency, but high spatial frequencies occur intwo bands disposed between the middle and the two ends. This overallfringe pattern may mask anomalies in the deformation pattern which occurin the areas of high fringe frequency.

in order to more carefully examine the area of the workpiece 10 in thevicinity of the fringe band of high density to the left of H6. 4, theangle which the reconstructing beam makes with the hologram may bealtered by moving the components 28 and 30 to positions 28' and 30 asshown in FIG. 1. Alternatively, the angle that the object beam makeswith the workpiece might be altered by an appropriate motion of themirror 40. Other changes could be made by moving the spatial filters 28or 38 so as to change the apparent sources of their beams. The effect ofsuch motions will be to shift the reconstructed wavefronts relative tothe real-time wavefronts in such a manner as to bring their overallslope into alignment at the area on the left of the workpiece. Asillustrated in FIG. 5, this decreases the spatial frequency in thatarea, as seen on the resultant image and reveals an anomaly 52 in thefringe pattern which results from a disbond between the skin 14 and thecore 16 in the area of the anomaly. At the same time the fringes to theright of the image have become even more closely spaced.

The area to the right of the image may be viewed in a more intensivemanner by displacing the reconstructing beam in the opposite directionso as to decrease the overall spatial frequency on the right in themanner shown in FIG. 6 on image 16"". This examination reveals no fringeanomaly.

Had these displacements of the reconstructing beam been made using aconventional hologram as opposed to an image-plane hologram, theresultant distortion of the reconstructed wavefronts would have madeanalysis impossible after movement of the reconstructing beam from theposition it occupied during the formation of the hologram.

The manner in which the present invention may be utilized with adouble-exposure technique is illustrated in FIG. 7. To make adouble-exposure holographic interferometric analysis of a workpiece 60,supported on a table 62, a laser 64 provides a coherent beam to a beamsplitter 66 which divides it into a reference beam that passes through aspatial filter 68 and is reflected by mirror 70 to a photographic plate72. The other beam from the splitter is reflected by mirror 76, througha spatial filter '76, and by a mirror 78 to the object 60. A lens 73forms an image-plane hologram of the object on the plate 72.

After the workpiece 60 is stressed, as by heat from the lamp till, asecond exposure of the same photographic plate is made. In this exposurethe position of the reference beam is changed by moving the mirror 66,the spatial filter 68 and the mirror 70 to positions 66', 68' and 70respectively. In practice the shift in position of the reference beambetween the two exposures of the photographic plate might be betterachieved by providing two sets of mirrors and spatial filters for thereference beam and suitable shuttering so that the first set is used forthe first exposure and the other set with the second exposure. In eitherevent, a second exposure of the photographic plate 72 is made, after theobject is stressed, and using a reference beam having a shifted positionwith respect to the first reference beam.

After development of the hologram, reconstruction requires the use oftwo reconstructing beams which initially share the same relationship toone another and also to the hologram as the original reference beams. Ifthe object has undergone no translation or deformation between the twoexposures, only a single object will be viewed. Minor magnitude overalland anomalous deformations will result in fringes on the apparentreconstruction. The fringe frequency over selected areas of the imagemay then be controlled by displacing one of the reference beams withrespect to the other in the same manner as is done in connection withthe realtime version. This modifies the angle of the two reconstructedwavefronts with respect to one another without in any way distorting thereconstructed image since image-plane holograms are utilized.

This technique combines all the advantages of realtime anddouble-exposure holography.

Having thus described our invention, we claim:

1. The method of performing holographic interferometric analysis of thedeformation of an object, comprising: forming an image-plane hologram ofthe object at a first time; deforming the object; reconstructing a firstwavefront from said hologram using a first source of coherent light;superimposing a second wavefront originating from the object at a secondtime, after the deformation, on said first wavefront using a source ofcoherent light which is coherent with said first source; and alteringthe phase of the reconstructed wavefront relative to the secondwavefront so as to modify the interference fringe density on thesuperimposed image, by moving the first source relative to the hologram.

2. The method of claim ll wherein said second wavefront is formed inreal time by light reflected from said object.

3. The method of performing holographic interferometric analysis of thedeformation of an object, comprising: forming a pair of coincidentimage-plane holograms of the object at respective first and second timeson a record media; deforming the object between said first and secondtimes; reconstructing two wavefronts of the object from said hologrammedia using first and second reference beams; and altering the phase ofone reconstructed wavefront relative to the second reconstructedwavefront so as to modify the interference fringe density on theresultant superimposed image by varying the angle of one of saidreconstructing beams relative to the record media.

4. The method of claim 3 wherein the process of formation of theimage-plane hologram employs a reference beam and the angle between thereference beam and the photographic media differs in the formation ofthe two holograms.

5. The method of performing holographic interferometric analysis,comprising: illuminating an object from a first coherent light source;forming a focused image of light reflected from said object on aphotographic media; simultaneously illuminating said media with lightcoherent with said first light source so as to record an interferencepattern between such coherent light and said image, on said media;developing said media to form a hologram; reconstructing a wavefrontfrom said hologram employing coherent light from a source; deformingsaid object; superimposing on the wavefront reconstructed from saidhologram a second wavefront consisting of light coherent with thereconstructing light source reflected from said object after saiddeformation; and modifying the angle of incidence of one of thereconstructing and second wavefront sources to adjust the phase of thereconstructed wavefront relative to the second wavefront.

6. The method of claim 5 wherein said second wavefront is generated byilluminating the object with the same light source which is used toreconstruct the wavefront from the hologram.

7. The method of performing holographic interferometric analysis,comprising: illuminating an object from a first coherent light source;forming a focused image of light reflected from said object on aphotographic media; simultaneously illuminating said media with lightcoherent with said first light source so as to record an interferencepattern between such coherent light and said image on said media;deforming said object; illuminating the object at a second time with asecond light source coherent with said first light source; forming asecond focused image of light reflected from said object on saidphotographic media superimposed on said first focused image;simultaneously illuminating said media with coherent light from saidsecond source to record an interference pattern between such coherentlight and said image on said media; developing said media to form adouble exposure hologram; illuminating said hologram so as toreconstruct said first and second superimposed wavefronts; and modifyingthe manner of illumination of said hologram to adjust the phase of thefirst reconstructed wavefront relative to the second reconstructedwavefront by varying the angle of one of said reconstructing wavefrontsrelative to the photographic media.

8. The method of claim 7 wherein the two holograms were formed employingreference beams which made different angles with the photographic media,two reference beams are employed to reconstruct the two wavefronts andthe position of the hologram relative to only one of the reference beamsis varied in order to adjust the position of the two reconstructedwavefronts relative to one another.

9. The method of performing holographic interferometric analysis of thedeformation of an object between two times, comprising: forming a firstimage plane hologram of the object at the first time on a photographicplate using a first reference beam making a first angle relative to saidphotographic plate; forming a second image plane hologram of the objectat a second time on said photographic plate, so as to be superimposed onsaid first hologram, using a second reference beam making a second anglerelative to said photographic plate; reconstructing the wavefrontsemanating from the object the first and second times from said hologramplate through use of first and second reconstructing beams which formdifferent angles relative to the hologram plate; and varying the angleof at least one of said reconstructing beams relative to the hologramplate in order to modify the interference fringe density on theresultant superimposed image.

10. The method of claim 9 wherein at least one of the reconstructingbeams formsthe same angle relative to the hologram plate as does one ofthe reference beams in the formation of the image plane holograms.

11. The method of claim 9 wherein lens means is em ployed between theobject and the photographic plate during the formation of the first andsecond image plane holograms in order to focus an image of the objectsubstantially at the plane of the photographic plate.

1. The method of performing holographic interferometric analysis of thedeformation of an object, comprising: forming an imageplane hologram ofthe object at a first time; deforming the object; reconstructing a firstwavefront from said hologram using a first source of coherent light;superimposing a second wavefront originating from the object at a secondtime, after the deformation, on said first wavefront using a source ofcoherent light which is coherent with said first source; and alteringthe phase of the reconstructed wavefront relative to the secondwavefront so as to modify the interference fringe density on thesuperimposed image, by moving the first source relative to the hologram.2. The method of claim 1 wherein said second wavefront is formed in realtime by light reflected from said object.
 3. The method of performingholographic interferometric analysis of the deformation of an object,comprising: forming a pair of coincident image-plane holograms of theobject at respective first and second times on a record media; deformingthe object between said first and second times; reconstructing twowavefronts of the object from said hologram media using first and secondreference beams; and altering the phase of one reconstructed wavefrontrelative to the second reconstructed wavefront so as to modify theinterference fringe density on the resultant superimposed image byvarying the angle of one of said reconstructing beams relative to therecord media.
 4. The method of claim 3 wherein the process of formationof the image-plane hologram employs a reference beam and the anglebetween the reference beam and the photographic media differs in theformation of the two holograms.
 5. The method of performing holographicinterferometric analysis, comprising: illuminating an object from afirst coherent light source; forming a focused image of light reflectedfrom said object on a photographic media; simultaneously illuminatingsaid media with light coherent with said first light source so as torecord an interference pattern between such coherent light and saidimage, on said media; developing said meDia to form a hologram;reconstructing a wavefront from said hologram employing coherent lightfrom a source; deforming said object; superimposing on the wavefrontreconstructed from said hologram a second wavefront consisting of lightcoherent with the reconstructing light source reflected from said objectafter said deformation; and modifying the angle of incidence of one ofthe reconstructing and second wavefront sources to adjust the phase ofthe reconstructed wavefront relative to the second wavefront.
 6. Themethod of claim 5 wherein said second wavefront is generated byilluminating the object with the same light source which is used toreconstruct the wavefront from the hologram.
 7. The method of performingholographic interferometric analysis, comprising: illuminating an objectfrom a first coherent light source; forming a focused image of lightreflected from said object on a photographic media; simultaneouslyilluminating said media with light coherent with said first light sourceso as to record an interference pattern between such coherent light andsaid image on said media; deforming said object; illuminating the objectat a second time with a second light source coherent with said firstlight source; forming a second focused image of light reflected fromsaid object on said photographic media superimposed on said firstfocused image; simultaneously illuminating said media with coherentlight from said second source to record an interference pattern betweensuch coherent light and said image on said media; developing said mediato form a double exposure hologram; illuminating said hologram so as toreconstruct said first and second superimposed wavefronts; and modifyingthe manner of illumination of said hologram to adjust the phase of thefirst reconstructed wavefront relative to the second reconstructedwavefront by varying the angle of one of said reconstructing wavefrontsrelative to the photographic media.
 8. The method of claim 7 wherein thetwo holograms were formed employing reference beams which made differentangles with the photographic media, two reference beams are employed toreconstruct the two wavefronts and the position of the hologram relativeto only one of the reference beams is varied in order to adjust theposition of the two reconstructed wavefronts relative to one another. 9.The method of performing holographic interferometric analysis of thedeformation of an object between two times, comprising: forming a firstimage plane hologram of the object at the first time on a photographicplate using a first reference beam making a first angle relative to saidphotographic plate; forming a second image plane hologram of the objectat a second time on said photographic plate, so as to be superimposed onsaid first hologram, using a second reference beam making a second anglerelative to said photographic plate; reconstructing the wavefrontsemanating from the object the first and second times from said hologramplate through use of first and second reconstructing beams which formdifferent angles relative to the hologram plate; and varying the angleof at least one of said reconstructing beams relative to the hologramplate in order to modify the interference fringe density on theresultant superimposed image.
 10. The method of claim 9 wherein at leastone of the reconstructing beams forms the same angle relative to thehologram plate as does one of the reference beams in the formation ofthe image plane holograms.
 11. The method of claim 9 wherein lens meansis employed between the object and the photographic plate during theformation of the first and second image plane holograms in order tofocus an image of the object substantially at the plane of thephotographic plate.